1. Field of the Invention
This invention relates to an adjusting shim used in a valve train in an internal combustion engine for an automobile.
2. Description of the Prior Art
A valve train in an internal combustion engine for an automobile is roughly constructed as shown in FIG. 1. Referring to FIG. 1, a reference numeral 1 denotes a cylinder head, 2 a cam, 3 a valve lifter, 4 an adjusting shim, 5 an intake and exhaust valve, 6 a valve seat, and 7 a valve spring. The valve lifter 3 is driven by the cam 2, and the displacement of the cam 2 is transmitted to the intake and exhaust valve 5. As may be understood from FIG. 1, the adjusting shim 4 is disposed between the valve lifter 3 and cam 2. This adjusting shim 4 is used to regulate a valve clearance. The adjusting shim 4 is usually formed out of a metal, and an adjusting shim formed out of a ceramic material so as to reduce the weight and improve the wear resistance is also available.
However, when an engine is provided with an adjusting shim consisting of a ceramic material, which is a brittle material, it is, for example, shifted down suddenly while the vehicle travels at high speed, so that the number of revolutions of the engine increases to exceed an allowable level, abnormal actions of a valve train, such as surging and jumping occur. In such a case, a high impact stress is imparted to the adjusting shim, so that the adjusting shim is broken. This causes the engine to be broken.
Further, a weight-reduced ceramic adjusting shim is used with a view to improving the practical power loss. However, since an adjusting shim accounts for a very small percentage of the inertial weight of a valve train, the weight reduction of a ceramic adjusting shim does not substantially bring about the improvement of a practical power loss.
Moreover, a ceramic material has an increased abrading power with respect to a cam 2 because of a large difference between the harnesses of the ceramic material and a cam. In some cases, the cam is worn greatly, and the weight reduction provides an adverse effect.